With the recent spread of thermal heads, the demand for heat-sensitive recording materials is increasing rapidly. In prepaid cards, in particular, which show expeditious progress in the fields of communication, transportation, distribution, and the like, many kinds of techniques for displaying magnetic information as visible information on the cards have come to be used. Such magnetic cards are being used extensively, and examples thereof include highway cards, prepaid cards for use in department stores, supermarkets, etc., JR orange cards (Trade mark), and the like.
However, since an area which can be used to display visible information is limited, there are cases where in the case of large-denomination prepaid cards, for example, renewed information concerning the balance becomes unable to be added any more. Such cards are normally replaced with reissued cards and this has been disadvantageous in cost, etc.
In order to overcome the above problem, studies are being made on a reversible recording material in which recording and erasion of information can be conducted repeatedly in the same area. Use of this material enables old information to erase and new information to display and, hence, avoids necessity of the issuance of a new card as a substitution for an old card in which renewed information cannot be displayed any more.
Conventionally proposed as such heat-sensitive materials in which information can be recorded and erased reversively are materials having a heat-sensitive layer comprising a resin matrix, such as poly(vinyl chloride), a vinyl chloride-vinyl acetate copolymer, a polyester, a polyamide, or the like, and an organic low molecular weight substance such as a higher alcohol, a higher fatty acid, or the like dispersed in the matrix (e.g., JP-A-54-119377, JP-A-55-154198, and JP-A-2-1363). (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
Formation and erasion of an image in such recording materials utilize a reversible change in transparency of the heat-sensitive layer by temperature changes. The Figure illustrates the transparency-opaqueness change for a reversible heat-sensitive recording material. The heat-sensitive layer in this recording material is in a transparent state in the temperature range of t.sub.1 --t.sub.1 ' and is in a milky and opaque state at temperatures of t.sub.1 ' or more. For heating the heat-sensitive recording layer, use of a thermal head is preferred particularly in the case where the recording layer has been formed on a magnetic card. That is, recording is, for example, conducted by making the initial state of the recording layer transparent and selectively heating the recording layer with a thermal head at a temperature of t.sub.1 ' or more to allow the heated area to turn milky and opaque, thereby to record a character or design. Alternatively, recording may be conducted by making the initial state of the recording layer milky and opaque and selectively heating the recording layer with a thermal head at a temperature in the range of from t.sub.1 to t.sub.1 ' to allow the heated area to turn transparent. Erasion of the thus-recorded image is accomplished by heating the recording layer with a heated roll, thermal head, or the like at a temperature of t.sub.1 to t.sub.1 ' in the case of the former recording technique and at a temperature of t.sub.1 ' or more in the case of the latter. However, the recording material disclosed in JP-A-55-154198 is defective in that since the temperature range in which the recording layer is transparent (t.sub.1 --t.sub.1 ': transparent-state temperature range) is as narrow as 1.degree. to 2.degree. C., it is almost impossible to control the heating temperature by means of a thermal head and, hence, the latter technique has been utterly unpractical. Although a recording material having a widened transparent-state temperature range has been proposed in JP-A-2-1363, the widened range has a width of only 2.degree. to 3.degree. C. and is hence still unpractical. In addition, the proposed recording material has had a permanence problem, for example, that because the transparent-state temperature range is in a low-temperature region, the recorded character or design disappears according to the ambient temperature. Further, since even a small amount of energy causes opaque state, there are attempts to use acrylic oligomers or various high temperature-boiling solvents (e.g. JP-A-63-104879, JP-A-63-107584, JP-A-63-179789 and JP-A-64-14078).
As described above, the conventional reversible heat-sensitive recording materials are disadvantageous in that the temperature range in which the heat-sensitive layer is transparent is narrow in any of these materials and that temperature control for making a partially opaque recording layer transparent or for forming a transparent image in a recording layer which is wholly opaque is not easy. Furthermore, in these recording materials, the temperatures at which the transparent state changes to the opaque state are as low as 70.degree. C. or less, so that there has been a drawback that transparent parts of the recording layer wholly or partly turn opaque during the storage of the recording material.
There has been another problem in the conventional reversible thermorecording techniques that after recording is conducted using a thermal head or the like and the recorded image is erased by a suitable means, an impact mark remains in the area where the recorded image was erased. This impact mark impairs the appearance of the recording material because light scattering occurs in the impact mark according to the viewing angle. There is a further problem that if recording of a visible image is conducted repeatedly in the same area using a thermal head, the reversible recording layer is severely deformed by the heat and pressure applied.